Under 50 pounds? Not much choice then, some simple MIDI Keyboards will already be 100 pounds if you want a good one.
Maybe an M-Audio Oxygen 25, which will cost you about 100 pounds.
But still I would invest in something more expensive, like the M-Audio Axiom 49 or a Novation in the same price range.
Lot more possibilities and you'll be able to use it much longer without thinking about buying a better MIDI controller.